Quick rules related question. Under the Sorcerer and Psyker archetypes it says that you may spend up to 500xp on various psychic powers. Is this from the xp you get at character creation for each respective class or is it just allowing you to choose up to 500xp worth of powers in addition to your starting xp of 500 for chaos marines and 1000 for humans.

It seems like it's bonus XP. Not much of a point in telling you you're allowed to spend 500 XP on psychic powers if that's possible anyway, is there?

Just the way it was worded made me take pause is all.

Yep, it's extra XP just for powers.

Much of it is the context it's in. That's not spent experience, that's in the section for "You have this. You have this. You have this OR this." If you can take one or another, you can spend xp to get the one you didn't choose at character creation. Same thing here. "You get anything up to 500xp already." Next you spend real XP.

Somewhat like Deathwatch I take it? Where you are given a list of powers to choose from right?

In the Psychic Powers section of the book. It says what "Disciplines" you can choose from on the Archetype page.